1. Field of the Invention
The present invention relates to a positive airway pressure support system for providing airway pressure release ventilation (APRV), and, more particularly, to a positive pressure support system and method of providing positive pressure support in an APRV mode that provides a back-up form of ventilation for use during episodes, such as episodes of apnea, where the patient stops breathing.
2. Description of the Related Art
Pressure support ventilation systems that provide a flow of breathing gas to an airway of a patient at an elevated pressure to treat a medical disorder are well known. One type of known pressure support ventilation is airway pressure release ventilation (APRV). APRV is a mode of medical ventilation that is often used to treat patients with acute lung injury. In APRV mode, a flow of breathing gas, such as air, is provided to a patient's airway at two different, alternating positive pressure levels (often referred to as PEEP High and PEEP Low) to provide ventilation and lung inflation while allowing the patient to spontaneously breath at both pressure levels.
The particular PEEP High and PEEP Low levels and the relative duration of each (i.e., the frequency of the changes from HIGH to LOW and back) are all parameters that are determined by the health care provider based on the particular therapy needs of the patient. For example, a patient may be placed on an APRV therapy wherein the PEEP High duration (referred to as “Time High”) is longer than the PEEP Low duration (referred to as “Time Low”) in order to facilitate ventilation while maintaining lung expansion. This is the more common type of APRV therapy that is provided. However, in some situations, particularly when a patient has less of a need for PEEP High lung inflation strategies, the patient may be placed on an APRV therapy wherein the Time High is shorter than the Time Low.
In current ventilators that provide APRV therapy (or a similar therapy that allows patients to breath spontaneously at two PEEP levels), if the patient becomes apneic (i.e., stops breathing), an apnea alarm is sounded and, in some ventilators, a back-up apnea ventilation mode is invoked. The settings for the back-up apnea ventilation mode are established by the clinician and do not attempt to preserve the initial intention of the base ventilator settings specific to APRV. In particular, in most current back-up apnea ventilation modes, the Time High ventilation is replaced with a fixed tidal volume or pressure at a relatively short inspiratory time. In situations where Time High is relatively long, this may not provide an adequate mean airway pressure for the patient due to the loss of the relatively long PEEP High setting.